Microelectronic or optoelectronic package having a polybenzoxazine-based film as an underfill material

ABSTRACT

Microelectronic and optoelectronic packaging embodiments are described with underfill materials including polybenzoxazine, having the general formula:

This is a Divisional Application of Ser. No.: 11/099,907 filed Apr. 5, 2005, which is presently pending, which is a Divisional Application of Ser. No.: 10/104,830, filed Mar. 22, 2002 which is now U.S. Pat. No. 6,899,960, Issued May 31, 2005.

BACKGROUND OF THE INVENTION

1). Field of the Invention

This invention relates to microelectronic and optoelectronic packages and, more specifically, to an underfill material in such packages.

2). Discussion of Related Art

A microelectronic or an optoelectronic package usually includes either a microelectronic semiconductor die or an optoelectronic die mounted to a package substrate. The package substrate provides rigidity to the overall package, and often also provides electronic interconnect between the die and another device. An underfill material is introduced between the die and the package substrate.

The semiconductor die, for example, may have conductive members on a lower surface thereof, and are positioned on top of an upper surface of the package substrate. The conductive members provide electric communication between the semiconductor die and the package substrate. The conductive members also mount the conductor die to the package substrate. The conductive members are, however, relatively fragile. Differences in coefficients of thermal expansion (CTE) between the semiconductor die and the package substrate cause relative shrinkage or expansion between the semiconductor die and the package substrate. The underfill material serves to reduce stresses on the conductive members due to such relative expansion or contraction.

A filler material is usually included in the underfill material. Filler loading is usually required to reduce shrinkage during curing, to control CTE, and to control moisture uptake. Filler loading tremendously increases the viscosity of the film at bonding temperature. The increased viscosity tremendously increases the bonding force per bump and reduces the wetting properties of melted film to all contact surfaces. The increase of bonding force per bump often cracks the chip during chip placement, and poor wetting properties of melted film adversely affect the joint integrity and introduces voids that shorten the joint fatigue life.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described by way of examples with reference to the accompanying drawings, wherein:

FIGS. 1 a-e are cross-sectional side views of polybenzoxazine-based films according to different embodiments of the invention; and

FIGS. 2 a-c are cross-sectional side views of electronic assemblies having polybenzoxazine-based underfill materials, according to different embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Microelectronic and optoelectronic packaging embodiments are described with underfill materials including polybenzoxazine, having the general formula:

wherein n can be any integer from 1 to 20 and is preferably an integer from 1 to 4 and is most preferably 1 or 2, and R₁ can be hydrogen, one or more groups selected from hydroxyls, one or more linear or branched alkyls of 1 to 80 and more preferably 1 to 10 carbon atoms, aromatics, alkyl substituted aromatics, aromatic substituted alkyls of 6 or 20 carbon atoms, mono or poly fluorine substituted alkyls of 1 to 20 carbon atoms, a mono or poly fluorine substituted compound having at least 1 aromatic ring and 6 to 20 carbon atoms, and phenolic compounds of 6 to 20 carbon atoms (for the purpose of this specification phenolic compounds may have more than one hydroxyl group as defined in chemical dictionaries) including poly phenolic compounds having on average 6 to 20 carbon atoms per phenol group. When n is 2, 3 or 4, R₁ can be selected from the connecting groups below:

Depending on whether phenolic or phenoxy repeat units are desired in the polybenzoxazine, it may be desirable that R₁ be ortho, meta, or para to the oxygen atom of the benzoxazine monomer of Formula A. Further, R₂ may be an alkyl of 1 to 40 carbon atoms, an aromatic, an alkyl substituted aromatic or aromatic substituted alkyl of 6 to 40 carbon atoms, mono or poly fluorine substituted alkyl of 1 to 20 carbon atoms, a mono or poly fluorine substituted compound having at least one aromatic ring and 6 to 20 carbon atoms or an amine of 1 to 40 carbon atoms including polyamines and aromatic, alkyl substituted aromatic, or aromatics substituted alkyls having 6 to 40 carbon atoms. Furthermore, each benzene ring, as shown by (R₃), where p is an integer from 0 to 3 and R₃ is as defined later, can have more than one substituent of the same structure or a mixture of the R₃ structures.

The variable m can be an integer from 0 to 5 and R₃ can be H or R₂. Preferably R₃ is not the amine or polyamine components of R₂. Preferably R₃ is an alkyl of 1 to 9 carbon atoms such as CH₃, C₂H₅, C₃H₇, or C₄H₉, or a mono or poly fluorinated alkyl of 1 to 9 carbon atoms such as CF₃, C₂F₅, or C₃F₇. These R₁ compounds are well known to those familiar with phenolic compounds. Generally R₁ can be any of the known connecting groups than interconnect two or more phenols. Known connecting groups refers to those which are present in commercially available phenols, are in experimentally available phenols, and phenols whose synthesis are described in the published literature. Examples of such phenols include:

As the cationically initiated ring opening polymerization of monofunctional benzoxazines generally results in linear polymers, it is desirable that at least 25 mole percent, more desirably at least 50 mole percent and preferably at least 75 mole percent or 90 mole percent of the R₁ groups are not an additional hydroxyl groups or a phenolic or polyphenolic compound and n in Formula A is 1. Also desirably at least 25 mole percent, more desirably at least 50 percent, still more desirably at least 75 or 90 mole percent of the R₂ groups are neither polyamines nor include additional benzoxazine compounds. These limitations are desirable for thermoplastic polybenzoxazines, but it is understood that if thermoset polybenzoxazines are desired, the amount of difunctional or polyfunctional benzoxazine monomers (those where n is 2 to 20 or R₂ is a polyamine or includes additional benzoxazine rings) could be higher. When high molecular weight thermoplastic polybenzoxazines are desired, desirably the number average molecular weight of the polybenzoxazine is at least 5,000 and more desirably at least 10,000.

As is well known, benzoxazine monomers are made from the reaction of three reactants, aldehydes, phenols, and primary amines by procedures using a solvent or known as solventless systems. U.S. Pat. No. 5,543,516 sets forth a generally solventless method of forming benzoxazine monomers. An article by Ning and Ishida in Journal of Polymer Science, Chemistry Edition, vol. 32, page 1121 (1994) sets forth a procedure using a solvent which can be used to prepare benzoxazine monomers. The procedure using solvents is generally common to the literature of benzoxazine monomers.

The preferred phenolic compounds are phenol or cresol, but can include diphenols (e.g., bisphenol-A), triphenols, etc., e.g., polyphenols, wherein each phenolic group in the phenolic compound has on average about 6 to about 20 carbon atoms per phenol group. The use of phenols with two or more hydroxyl groups reactive in forming benzoxazines may result in branched and/or crosslinked products. The groups connecting the phenolic groups into a phenol can be branch points or connecting groups in the polybenzoxazine.

The aldehydes used to form the benzoxazine can be any aldehyde, but preferably the aldehydes are those having from about 1 to about 10 carbon atoms, with formaldehyde being highly preferred. The amine used to form the benzoxazine can be an aromatic amine, an aliphatic amine, an alkyl substituted aromatic, or an aromatic substituted alkyl amine. The amine can also be a polyamine, although the use of polyamines will, under some circumstances, yield polyfunctional benzoxazine monomers. Polyfunctional benzoxazine monomers are more likely to result in branched and/or crosslinked polybenzoxazines than monofunctional benzoxazines, which would be anticipated to yield thermoplastic polybenzoxazines.

The amines generally have from about 1 to about 40 carbon atoms unless they include aromatic rings, and then they may have from about 6 to about 40 carbon atoms. The amine of di or polyfunctional may also serve as a branch point to connect one polybenzoxazine to another.

In the past, thermal polymerization has been the preferred method for polymerizing benzoxazine monomers. The temperature to induce thermal polymerization is typically varied from about 150 to about 300° C. The polymerization is typically done in bulk, but could be done from solution or otherwise. Catalysts, such as carboxylic acids, have been known to slightly lower the polymerization temperature or accelerate the polymerization rate at the same temperature.

Cationic polymerization initiators described in this invention have been found to result in polymerization of benzoxazine monomers at temperatures as low as cryogenic temperatures. Preferred temperatures are from about −100° C. to about 250° C., and most preferably between about −60 and 150° C. for ease of handling the reactants and products. Some of the cationic initiators, e.g., PCl₅, form repeating units from the benzoxazine monomers, that include a salt of the amine. These repeating units have better solubility in polar solvents, e.g., water, than similar repeating units without the amine salt. The initiators of the current invention can be used either in the benzoxazine melt or in the presence of solvent, allowing the solvent content to be from 0% to nearly 100%. Many solvents can be used in cationic polymerizations, and their selection is known by those skilled in the art of cationic polymerization.

The polymers from the cationically initiated polymerization of benzoxazine are useful as molded articles with good thermal stability and/or flame resistance, such as molded circuit boards, flame resistant laminates, or other molded articles, and is a source of precursor to high temperature resistant chars. The common uses for high temperature resistant chars include aircraft brake discs, equipment for sintering reactions, and heat shields or heat shielding material. The polymers which include repeating units having amine salts can be used in applications for partially or fully water soluble polymers such as viscosity control agents.

Generally, given optimal reaction conditions, cationic initiators can polymerize benzoxazine monomers or oligomers. These include H₂SO₄, HClO₄, BF₃, AlCl₃, t-BuCl/Et₂AlCl, Cl₂/BCl₃, AlBr₃, AlBr3.TiCl₄, I₂, SnCl₄, WCl₆, AlEt₂Cl, PF₅, VCl₄, AlEtCl₂, and BF₃Et₂O. Preferred initiators include PCl₅, PCl₃, POCl₃, TiCl₅, SbCl₅, (C₆H₅)₃C⁺(SbCl₆)⁻, or metallophorphyrin compounds such as aluminum phthalocyanine chloride, which are all known to result in similar polymers from cationically initiated polymerization of unsaturated monomers. Methyl iodide (a covalent initiator), butyl lithium (an ionic initiator), and benzoyl peroxide (a radical initiator) were not effective at polymerizing experimental benzoxazine monomers.

Methyl tosylate, methyl triflate, and triflic acid appear to cationically polymerize the experimental benzoxazine monomers, although the polymers did not precipitate from solution. Typically, each initiator initiates a polymer with from about 3 to about 1,000 to 3,000 repeat units, so the amount of initiator needed on a mole percent basis relative to the monomer is small. However, an additional initiator is needed to compensate for loss due to adventitious moisture and other reactants that deactivate cations, that may be present in the monomers solvents, etc. Desirably about 0.001 to about 50 mole percent initiator based upon the monomer and more desirably from about 0.01 to about 10 mole percent initiator is used for these cationically initiated polymerizations.

EXAMPLES

Several factors have been found to change the structure of the benzoxazine polymers prepared by cationic polymerization. These factors include polymerization temperature, the particular cationic initiator, and competing reactivity between the ortho-carbon of the benzene ring and the basic nitrogen atom of the oxazine ring.

For example, the following results were obtained with PCl₅ as the initiator at −60° C. and 40° C.:

Thus, the formation of a phenolic repeating unit having an amine salt therein occurs preferentially to this kind of monomer (monoparasubstituted benzene ring other than the two sites occupied by the oxazine ring) at low temperatures (e.g., −60° C.) with a PCl₅ initiator. A phenoxy repeating unit occurs more frequently at 40° C. than at −60° C., but still is not the exclusive repeating unit. The amine salt form of the phenolic repeating unit causes an increased water solubility. Further, at −60° C. the availability or reactivity of an ortho CH position also affects the relative ratio of phenolic to phenoxy repeat units. Steric hindrance around the ortho CH position or a substituent in the ortho Cl position favors a phenoxy structure from the benzoxazine, while the absence of steric hindrance and absence of an ortho substituent favors a phenolic structure. A unified theory to explain the above is that there is a competition between the reactivity of the aromatic carbon (ortho with respect to the oxygen) and the basic nitrogen atom of the (O—CH₂—N(R₂)CH₂). When substitution on the aromatic ring is in such a way that it reduces the reactivity of the ring ortho carbon, the protonation or alkylation of the nitrogen dominates and a phenoxy structure results. However, if the ortho CH position (with respect to the O) is open and no bulky substituent offers steric hindrance, the ring reactivity dominates and a phenolic structure at low temperatures is the result. In summary, it is anticipated that at −60° C., if i) aromatic ring reactivity dominates, then a phenolic repeating structure is obtained, and if ii) oxazine ring reactivity dominates, then a phenoxy repeating structure is obtained, and that monomers given below will result in an increased probability of the repeating structures given. Monomer Preferred Repeating Structure

Phenolic structure

Phenoxy structure

Phenoxy structure

Mixture phenolic and phenoxy

As phenolic repeat units from benzoxazine are now available from lower temperature cationic polymerizations, it is desirable to distinguish the resulting polymer from thermally polymerized benzoxazine polymers. Desirably the number average molecular weight of polymers from cationically polymerized monofunctional benzoxazine monomers are at least 2,000 or 5,000. Desirably the cationically polymerized benzoxazine when both phenolic and phenoxy structures result has at least 2, 5, 10, or 15, or at least 50, 80, or 90 mole % repeating units of the phenoxy structures. However, the pure phenolic structure when the molecular weight is above 2,000 or 5,000 is not excluded.

Five benzoxazine monomers were used for most of the experimentation on cationically initiated polymerization. They were BA-a monomer and C-m monomer. Different chemical structures for two of the monomers are shown below:

BA-a monomer is made from the reaction of bisphenol A, formaldehyde, and aniline. C-m monomer is made from the reaction of cresol, formaldehyde, and methylamine. C-m monomer was prepared according to the procedure set forth by Ning and Ishida in Journal of Polymer Science, Chemistry Edition, vol. 32, page 1121 (1994). The polymerization solvents used were 1,2-dichloroethane, 1,2-dichlorobenzene, chloroform, or deuterated chloroform. All the solvents were dried by conventional methods and distilled under argon. The cationic initiators used in Table 1 were purchased from Aldrich Chemical Company and used without further purification. All solvents and reactants were stored in a dry box unless refrigeration was required. The moisture was usually less than 1 ppm in the dry box. Table 1 below shows polymerizations, which were ran for about 20-80 hours using the specified variety of cationic and other initiators. Table 1 sets forth the results of polymerizations of BA-a monomer. While run numbers 8, 9, and 11 did not yield any insoluble polymer, the solutions turned deep red and a tough red polymer was recovered when the solvent was evaporated. The initiator amount was typically about 5 mole percent based upon the amount of benzoxazine monomer: TABLE 1 ChCl₃ insoluable as wt. % based Run No. Initiator Temperature on total monomer weight 1 Phosphorus pentachloride 20 56.2 (PCl₃) 2 Phosphorus trichioride (PCl₃) 20 19.6 3 Phosphorus oxychioride 20 15.0 (POCl₃) 4 Titanium (V) chloride (TiCl₅) 19.4 5 Triphenylcarbenium 20 8.7 antimonatehexachloride [(C₆H₅)₃ C + (SbCl₆)⁻] 6 Antimony pentachioride 53 10.1 (SbCl₅) 7 Antimony pentachloride/ 20 12.3 oxetane (Sb Cl₅) 8 Methyl tosylate (MeOTs) — — 9 Methyl triflate (MeOtf) — — 10 Aluminum phthalocynanine 20 20.4 chloride (metallophorphyrin) 11 Triflic acid — — 12 Borontrifluoro dietherate 53 0 13 Borontrifluoro dietherate/ 20 0 promoter 14 p-tolyl triflate 53 0 15 Methyl iodide covalent 53 0 initiator 16 Butyl lithium (anionic) 20 0 17 Benzoyl peroxide (free 110 0 radical)

TABLE 2 Char yield at 800° C. Initiator Tg by DSC (° C.) by TGA (%) PCl₅ 215 50.26 PCl₃ 216 48.93 POCl₃ 210 50.53 TiCl₅ 222 61.78 Metallophorphyrin 186 43.59 MeOTf 193 31.48 MeOTs 142 28.06 Triflic acid — 31.29 (C₆H₅)₃C⁺(SbCl₆)⁻ — — SbCl₅ — — Thermal-cured (Cntrl) 177 28.56

Several of the polymers polymerized according to the condition set forth in Table 1 were analyzed by thermogravimetric analysis (TGA) or differential scanning calorimity (DSC). The glass transition temperature (Tg) is used to characterize a particular polymer and microstructure. The char yield is affected by many factors, a high char yield is typically desired for precursors for char or for non-flammable polymers. As can be seen in Table 2, the first four cationic initiators yielded polymers from benzoxazine having significantly higher Tg's than the last sample, which was a control (thermally cured) polybenzoxazine from the same monomer. The char yield from the polymers initiated with the first four cationic initiators varied from about 44 or 49 to about 62 weight percent based upon the weight of the initial starting polybenzoxazine. This was significantly higher than the control, which yielded only about 29 weight percent char under identical conditions. The next three cationic initiators (5-7) yielded polymers with slightly different Tg's than the control. The char yield from the fifth through the seventh cationic initiators was also different than that of the control, although the difference from the control amount was not as significant as with the first four cationic initiators. It is to be noted that the polymer from the methyl tosylate, methyl triflate, and triflic acid had a different color (red) than the polymers from the first four cationic initiators.

The polymers from the cationic polymerization of benzoxazine monomer BA-a were analyzed by Fourier transform infrared spectroscopy along with a thermally cured polybenzoxazine from the same monomer. These results indicated substantially similar infrared spectra when bisphenol A-based benzoxazine was used. The polymers compared to the monomer showed the dramatic change in the peaks in the region of 1,000 to 1,350 cm⁻¹, arising from the CH₂ wagging (1327 cm⁻¹ and 1305 cm⁻¹), C—O—C asymmetric stretching (1233 cm⁻¹), C—N—C asymmetric stretching (1159 cm⁻¹), and C—O—C symmetric stretching (1031 cm⁻¹) of the oxazine ring, respectively, as well as the decrease of the peak resolution, indicate the opening of the oxazine ring and the polymerization of the monomer into oligomers and polymers.

There are significant spectral differences between the thermally polymerized polybenzoxazine and the PCl₅ initiated polybenzoxazines. In the spectrum of BA-a monomer the peaks centered at 1500 cm⁻¹ and 949 cm⁻¹ are major characteristics of the tri-substituted benzene ring in the benzoxazine structure, corresponding to the in-plane C—C stretching and the out-of-plane C—H deformation of tri-substituted benzene ring, respectively. These two peaks almost disappear in the thermally polymerized polybenzoxazine spectrum, meanwhile a new peak centered at 1489cm⁻¹, representing tetra-substituted benzene ring mode, appears. The results are in accordance with the well-accepted thermal polybenzoxazine structure shown as below:

However, with some cationic initiators, here PCl₅ has been chosen as an example, these two peaks at 1500 cm⁻¹ and 949 cm⁻¹ remain unaffected except for the peak broadening, which indicates that in the polybenzoxazine structure obtained, the majority of the benzene rings should still be trisubstituted instead of tetrasubstituted. Further NMR results are in support of this hypothesis.

Polymers for size exclusion chromatographic (SEC) analysis were cationically polymerized from the BA-a monomer using chloroform as a solvent at about 23° C. using PCl₅ initiator and a mole ratio of 1:100 initiator:monomer. Portions of the reactants were withdrawn after 1, 2, 2.5, 3.5, 5, and 7 hours of reactions and poured into an excess of methylol to cause precipitation. The precipitated material was recovered and analyzed by SEC for differences in molecular weight as a function of reaction time. As is well-known in SEC analysis, short retention times indicate higher molecular weight, while long retention times indicate lower molecular weight. A peak at 29.3 minutes was associated with the benzoxazine monomer. A new peak occurred between 21 and 29 minutes as the polymerization occurred. Simultaneously the peak at 29.3 minutes decreased in size. This is consistent with theories on polymerization.

An NMR experiment was conducted comparing polymer from a cationic polymerization, using PCl₅ as the initiator in deuterated chloroform as a solvent, to a thermally polymerized polymer. The reaction temperature for the cationic polymerization was 40° C., the reactor was a sealed NMR tube, and the material was evaluated after 10 minutes, 30 minutes, 1 hour, and 2 hours to see which NMR peaks were being generated or depleted as the monomer was cationically polymerized. The monomer peaks at 2.6, 3.9, and 4.75 ppm gradually disappeared, while new peaks at 1.95 and 3.65 ppm appeared, and a broadening of the peak at 2.2 ppm occurred indicating less molecular rearrangement occurred ongoing from monomer to polymer under cationic conditions. Based on these results (proton NMR) along with carbon ¹³NMR and FTIR results, the repeat structure of the thermally polymerized polymer in these studies is theorized to be predominantly tetrasubstituted phenolic structure, e.g.,

while the cationically polymerized samples is predominantly the phenoxy repeating units of the structure

In conclusion, the experimental data indicates the cationically initiated polymerization benzoxazine monomers can occur at substantially lower temperatures than thermal polymerization and produces polymers of different microstructure than thermal polymerizations. Further, these polymers can have substantially higher char yields 62\29 or an increase of 200 percent over the char yield of the thermally polymerized polymers.

The polymers from benzoxazines are useful as precursors for char yielding material (e.g., precursors to aircraft brake pads). They are also useful as temperature and flame resistant polymers for electrical components, planes, cars, buses, etc.

A polybenzoxazine material has the following characteristics, making it desirable as a microelectronic or optoelectronic underfill material:

-   -   (i) Low moisture uptake;     -   (ii) Low volume shrinkage during curing;     -   (iii) Good film-forming properties (i.e., thickness control and         tackiness at room temperature);     -   (iv) A low coefficient of thermal expansion (CTE).

The following table compares the properties of a typical polybenzoxazine compound with a typical epoxy: TABLE 3 Typical Polybenzoxazine- Property Based Film Typical Epoxy Curing shrinkage (%) 0 or expansion 3-4 Moisture uptake (%) 1.5  3 Modulus (GPa) 4 2-3 Tensile strength (MPa) 130 120 Tg (° C.) 170 150 Impact strength (J/m) 30  30 CTE (ppm/° C.) 55  65

Different film structures can be designed to meet application requirements. Film structures according to embodiments of the invention are shown in FIGS. 1 a-1 e. In each case, a film 2 a-e is provided between two removable sheets 1. FIG. 1 a illustrates a film 2 a, which is a polybenzoxazine-based film as hereinbefore described. FIG. 2 b illustrates a polybenzoxazine-based film 2 b including uniformly-dispersed filler particles 4. FIG. 2 c illustrates a polybenzoxazine-based film 2 c including anisotropic electrically conductive particles 3. FIG. 2 d illustrates a polybenzoxazine-based film 2 d including electrically conductive columnar components 5. FIG. 2 e illustrates a polybenzoxazine-based film having multiple layers 2 e with components 3 therein. The components 3 typically have a lower CTE than the polybenzoxazine material of the layers 2 e. More of the components 3 are located in an upper one of the films 2 e than in lower ones of the films 2 e so that the upper film has a lower coefficient of thermal expansion due to the higher number of components 3 than lower ones of the films 2 e.

FIG. 2 a illustrates an optoelectronic package 8 a and an optoelectronic die 9 which is attached to a package substrate 6 by means of an adhesive film 7. The film 7 may, for example, be the film 2 a illustrated in FIG. 1 a. Film 7 shrinks by a relatively small amount during curing so that the position of the die 9 is substantially maintained relative to the substrate 6. Such a feature is desirable when coupling light from another component, which is at a predetermined distance from the package substrate 6, to a feature 9 a on the die 9. A microelectronic package can be assembled n a similar manner by replacing the optoelectronic die in 9 with a microelectronic die.

FIG. 2 b illustrates an electronic assembly 8 b having a semiconductor die 13 attached to a package substrate 10 with a film 11 that may be the same as the film 2 d illustrated in FIG. 1 d. The film 11 thus includes a plurality of columnar members 14. Each member 14 has an upper end in contact with a respective contact 12 on the die 13, and a lower end in contact with a respective pad 15 of the package substrate 10. The die 13 is in electric communication with the package substrate 10 through the members 14. A lower surface of the die 13 adheres to an upper surface of the material 11, and an upper surface of the package substrate 10 adheres to a lower surface of the material 11. The material 11 electrically insulates the members 14 from one another.

FIG. 2 c illustrates an electronic assembly 8 c including a semiconductor die 20 which is attached to a package substrate 16 through a film 18. The die 20 has a plurality of contacts 21 on a lower surface thereof, and the package substrate 16 has a plurality of pads 24 on an upper surface thereof. The film 18 includes a polybenzoxazine material 19 having an upper surface attached to a lower surface of the die 20, and a lower surface attached to an upper surface of the package substrate 16. The film also has columnar members 22 in the material 19. Each member 22 establishes an electrical path between a die and its respective substrate, and has a respective upper end contacting a respective one of the contacts 21 and a respective lower end contacting a respective one of the pads 24. The film 18 also has a plurality of filler particles 17 in the material 19. The particles 17 have a lower CTE than the material 19, and thus decrease the overall CTE of the film 18. More of the particles are included near the die 20 than near the package substrate 16, so that the CTE of the film 18 is lower near the die 20 than near the package substrate 16. The die 20 has a lower CTE than the package substrate 16. The CTE of the film 18 closely matches the CTE of the package substrate 16 near the package substrate 16, and closely matches the CTE of the die 20 near the die 20. A similar package can be constructed without columns in the film. It may, for example, be possible to locate the film of FIG. 1 e on a substrate having bumps or other contacts thereon, or a substrate without bumps but with bumps on the die. It may also be possible to use the film of FIG. 1 e between a die and a substrate where both die and substrate have facing bumps, so that the bumps on both the die and the substrate penetrate the film.

Most of polybenzoxazine-based film monomers are solids at room temperature. But they normally have low melt viscosity at slightly elevated temperatures such as 70-80° C. Cured pure polybenzoxazine-based film materials also have a lower CTE than a typical epoxy material, which allows lower filler loading to reach the same CTE value as a typical epoxy. The low melt viscosity and lower filler loading facilitate the material flow and wet on contact surfaces, and therefore provide a wider process window. Additionally, the lower moisture uptake of the polybenzoxazine-based film material is expected to have better performance of a finished package during moisture testing. There is also a slight volume increase during the cationic ring opening polymerization of the polybenzoxazine material. This volume expansion can effectively reduce residual stresses during the cooling of the entire package.

Considering that polybenzoxazine-based film monomers can be easily made through Mannich reaction from a phenolic derivative, formaldehyde, and a primary amine, it can be seen that a polybenzoxazine-based film can be synthesized from a wide selection of raw materials consisting of phenolic derivatives and primary amines. The molecular structure of polybenzoxazine-based films offers superb design flexibility to allow for the properties of the cured materials to be controlled for the specific requirements of a wide variety of individual applications.

While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the current invention, and that this invention is not restricted to the specific constructions and arrangements shown and described since modifications may occur to those ordinarily skilled in the art. 

1. A method of constructing an electronic assembly, comprising: adhering an electronic die to a substrate with a film including polybenzoxazine.
 2. The method of claim 1, wherein the film includes a plurality of electrically conductive members, each member electrically connecting a respective contact on the die with a respective pad on the substrate.
 3. The method of claim 1, wherein the film is formed by reacting a cationic polymerization initiator with 2H-1,3,-dihydrobenzoxazine monomers at a temperature from about −100° C. to about 250 C.
 4. The method of claim 3, wherein the cationic polymerization initiator comprises PCl₅, PCl₃, POCl₃, TiCl₅, (C₆H₅)₃C⁺(SbCl₆)⁻, SbCl₅, methyl triflate, methyl tosylate, triflic acid, or aluminum phthalocyanine chloride or combinations thereof.
 5. The method of claim 3, wherein the 2H-1,3,-dihydrobenzoxazine monomers forming the polymer comprise at least 25 mole percent of one or more monomers of the formula

wherein R₁ individually is hydrogen, one or more groups selected from alkyls of 1 to 10 carbon atoms, an aromatic, alkyl substituted aromatics, or aromatic substituted alkyls of 6 to 20 carbon atoms, aromatic ring and 6 to 20 carbon atoms mono or poly fluorine substituted alkyls of 1 to 20 carbon atoms, mono or poly fluorine substituted compound having at least one or a benzoxazine from a phenolic compound of 6 to 20, n is from 1 to 4; wherein each R₂ is an alkyl of 1 to 10 carbon atoms, an aromatic, alkyl substituted aromatic, or aromatic substituted alkyl of 6 to 20 carbon atoms, or an amine of 1 to 10 carbon atoms, or a benzoxazine of 9 to 20 carbon atoms; and wherein R₃ is H or R₂ and p is an integer from 0 to
 3. 6. The method of claim 3, wherein at least 20 mole % of the repeat units of the polymer arc of structure I.
 7. A method of constructing an electronic assembly, comprising: locating a film between a substrate and an electronic die; and curing the film by first heating the film and then allowing the film to cool, the film having less than 0.2% shrinkage during curing.
 8. The method of claim 7, wherein the film expands by at least 0% during curing.
 9. The method of claim 7, wherein the film has a modulus of at least 1 GPa, a strength of at least 80 MPa, and an impact strength of at least 15 J/m.
 10. The method of claim 9, wherein the film has a modulus of at least 4 GPa, a strength of at least 130 MPa, and an impact strength of at least 30 J/m.
 11. The method of claim 7, wherein the film, without any filler material, has a CTE of less than 70 ppm/° C.
 12. The method of claim 7, wherein the film includes a polybenzoxazine polymer. 